Compositions comprising a polyphenylene ether resin and a styrene resin are well known as useful engineering thermoplastics, for molding, extrusion and the like. They are described in Cizek, U.S. Pat. No. 3,383,435, which is incorporated herein by reference.
Such compositions are normally flammable, particularly if high proportions of styrene resin are present, and aromatic phosphate compounds, e.g., triphenyl phosphate are used to retard or eliminate flammability. Haaf, U.S. Pat. No. 3,639,506, also incorporated herein by reference, discloses that triphenyl phosphate has a tendency to reduce physical properties and describes the use of combinations of aromatic phosphates and aromatic halogen compounds to flame retard the composition, without markedly lowering resistance to distortion by heat.
Pentaerythritol esters of phosphoric acid have been reported in Wahl et al, U.S. Pat. No. 3,090,799, to be generally superior as plasticizers and as flame retardants for synthetic resins. However, it is disclosed that from 5 to 40 parts of the said phosphate per 100 parts of the resin is the proper amount to use, and all of the working examples appear to fall in the range of 5 to 43 parts of the said cyclic phosphate per 100 parts of the combination, by weight.
It has now been discovered that cyclic phosphate compounds by themselves are effective non-plasticizing flame retardant additives for the compositions of polyphenylene ethers and styrene resins, at an unexpectedly lower concentration. Moreover, such cyclic phosphate materials are just as effective in flame retardance in this system as triphenyl phosphate and provide compositions with substantially the same impact resistance as the polymer composition itself.